In radiation therapy a high dose of radiation is applied to cancerous tissue in order to destroy it or to at least slow its growth. It is critical to accurately apply the radiation so that the target receives most of the dose and surrounding healthy tissue is spared. For this reason, imaging systems have been incorporated into radiation treatment systems in order to determine if the treatment target is in the correct location when the patient is positioned relative to the treatment beam. Also, since the target may move due to respiration and other physiological processes, even when the patient remains stationary, it is desirable to obtain images of the target immediately before and during radiation treatment. A number of inventions have been directed at imaging the target before or during radiation treatment.
One means of obtaining an image of the target region is to use the therapeutic beam as the imaging source to create a portal image. The drawback of this approach is that portal images have very low contrast due to the high energy of the therapeutic beam. U.S. Pat. No. 5,471,516 solves this problem by incorporating a lower energy diagnostic X-ray source into the treatment head to produce a diagnostic portal image.
U.S. Pat. No. 6,888,919 discloses a radiotherapy machine with a first pivotal gantry that contains a therapeutic radiation source and a second pivotal gantry that contain opposing diagnostic X-ray source and imager. The diagnostic radiographic imaging unit can be rotated around the patient to provide a data set that can be used to produce a three-dimensional reconstruction of the region around the target.
U.S. Pat. No. 6,778,850 discloses a radiotherapy machine with two diagnostic X-ray sources mounted above the treatment couch that capture images of the target region from different perspectives. The simultaneously captured images are used to create a three-dimensional mapping of the target region.
U.S. Pat. No. 6,914,959 discloses a combined radiotherapy and CT imaging system. The CT system can be used to capture planning images and images of the target during treatment. In this way, the patient need not be moved between the planning and treatment phase.
A shortcoming of present methods of imaging a radiation therapy target is that they do not enable the capture of an image of the target region in which detectability of the target is optimal for all positions of the treatment couch. Furthermore, some of the present methods require that the diagnostic source be moved in order to capture several images of the target region that may be used to reconstruct a three-dimensional image. This can preclude near real-time target localization.
An object of the present invention is to provide an imaging system that can capture images of a radiation therapy target region, using desired capture conditions for which the radiation target can be detected, without the use of internal markers. Another object of the present invention is to maintain the desired capture conditions when the treatment couch is moved. Another object of the present invention is to obtain images in which the target is detectable without the need to move the imaging source or detector during the image acquisition process so that the target can be localized in near real-time.